Fuel cells are used in many applications. They are for example used as a source of energy in electric motor vehicles, or for recharging batteries, for example for recharging batteries of portable devices (telephones for example).
Non-isolated power supplies with DC DC splitting, also called converters or choppers, are usually used for converting the direct-current voltage supplied by the cell to another direct-current voltage which may be higher or lower and which may have the same polarity or reverse polarity, depending on the topology of the power supply. The power handled by the power supply depends on the output load, that is to say the application that is using the voltage supplied at the output.
The invention is focused more particularly on splitting power supplies of the interlaced cells type. These power supplies make it possible to reduce the ripple of current at the input of the power supply and at the output. Reducing the input and output ripple is a criterion of quality of a splitting power supply. Each cell is a converter. The concept of interlacing arises from the fact that the cells supply in phase shift the power to an output capacitor which can be common to the cells or specific to each cell.
The interlaced cells are each a DC DC converter. They all have the same topology which is chosen depending on the range of power and on the gain that is sought for the application in question. The various well known power supply topologies that are used in these power supplies are the voltage step-up, called “boost”, voltage step-down, “buck”, voltage inverter and step-up/step-down, “buck-boost”, or “Cuk” topologies from the name of its inventor, voltage step-up/step-down or SEPIC (“Single ended primary inductor converter”).
In a manner common to these various topologies, each DC DC conversion cell is an electric circuit that is usually tripolar with one input terminal, one output terminal and one common terminal. This electric circuit comprises at least one switch and a diode and an energy storage element, typically an inductor. The input voltage is applied between the input terminal and the common. An output capacitor is connected between the output terminal and the common. Energy is transferred from the input to the output by means of the energy storage element, which stores the energy and then restores it at the rate of switching of the switch in the open state and the closed state. Voltage regulation is performed by the conduction time (closed state) of the splitting switch.
The switch S is usually produced by a field effect transistor. That is why reference is made without distinction to a switch in the open state, or off-state, and in the closed state, or on-state. Typically in the range of input and output voltages from a few volts to several thousands of volts, use is preferably made of a transistor of the IGBT (Isolated Gate Bipolar Transistor) type, capable of withstanding high voltages at its terminals. This technological solution makes it possible to ensure the reliability of the converter while minimizing the cost of the components.
The operation of a cell, and that of a power supply with interlaced cells will be reviewed with respect to FIGS. 1a to 1c, giving details of the structure and the operation of a BCi cell, and FIG. 2 giving details of a power supply comprising three cells of this type, BC1, BC2 and BC3.
The BCi conversion cell (FIG. 1a) is a tripolar cell with a star topology: a switch S, an inductor L and a diode D each form a branch of the tripole. All the branches start from a common node A and their termination forms one of the three terminals of the tripole.
In the example illustrated, the topology of the BCi cell is that of a BC voltage step-up (boost) converter. The switch S is connected between the node A and the common terminal B3. The diode has its anode connected to the node A, and its cathode connected to the output terminal B2. The inductor L is connected between the input terminal B1 and the node A.
The splitting switch S is controlled in the usual manner in pulse-width modulation mode by a pulse signal with a constant frequency f which places it alternately in a closed state, for a closure time marked at and an open state, for an open time lasting t-αt, α being the cyclical ratio between the time the switch is closed and the complete period of the cycle (t=1/f).
The two operating phases of such a converter, which correspond to the two states, closed and open, of the switch S, are as follows:                when the switch S is closed: the inductor L is in parallel on the input voltage source and the current increases in the inductor. This is the energy storage phase. The diode D is then disabled. The equivalent wiring diagram is illustrated in FIG. 1b.         when the switch S is open, the inductor L is in series with the input voltage source Ue. The current passes through the inductor L and the diode D and the output capacitor Cs is charged. This is the energy transfer phase. The equivalent wiring diagram is illustrated in FIG. 1c. The voltage at the terminals of the output capacitor Cs becomes higher than the input voltage.        
The output voltage level depends in practice on the durations of the open and closed times of the switch. If the splitting power supply works at a constant frequency f and in continuous conduction mode (that is to say that the current passing through the inductor is never cancelled out), the output voltage Us is equal to α*Ue.
FIG. 2 illustrates a splitting power supply with n interlaced cells charging a single output capacitor Cs.
In the example, n=3 cells BC1, BC2, BC3 that are identical (L, S, D) and in parallel: their terminals B1 are connected together; their terminals B2 are connected together; their terminals B3 are connected together. The power supply comprises a single output capacitor Cs connected between the output terminal B2 and common terminal B3 of each cell. The input voltage Ue is applied between the input terminal B1 and common terminal B3 of each cell.
In this figure, the cells have the same converter topology, i.e. voltage step-up (boost), as the cell shown in FIG. 1a. 
The n=3 splitting switches S are each controlled as indicated with reference to FIGS. 1a to 1c, at the same splitting frequency f, but the various paths are offset relative to one another by a time shift of fixed duration, corresponding to a phase shift between each cell of 2π/nf. The frequency of the currents and of the voltages seen by the load is therefore n times greater than that which is obtained with a single cell. The input ripples and the ripples in the output capacitor are reduced.
The invention focuses notably on an enhancement of these power supplies with n interlaced cells, an enhancement which advantageously makes it possible to ease the placement of the various components with regard to evacuating heat and reducing or even cancelling the voltage or current ripple at the input and the output due to the inductances of wiring caused by the connections to be made between the various elements of the power supply. These are notably the wiring inductances Lw in series between the switch S and the diode D of each cell which disrupt the charge transfer loop (switch S, diode D, output capacitor CS). When the power supply includes one output capacitor per cell, these also are the wiring inductances between the output capacitors (not illustrated in FIG. 2). In this enhancement, the splitting switch S of each cell is placed in a resonant circuit which is shown schematically by a simple rectangle referenced 10 in FIG. 2, and which contains the switch S.
This resonant circuit 10 makes it possible to lock the splitting switch at zero current in order to transfer from the storage phase to the energy transfer phase. The locking occurs also at zero voltage. The technical effect produced by a locking without loss is notably to ensure that the wiring inductances of the charge transfer loop (T, D, C) have no effect on the efficiency of the power supply. Therefore, even though the wiring inductances are high, they no longer have an effect in the conversion.
FIG. 3 gives for one cell of the supply details of an embodiment of this resonant circuit 10 applicable to the power supply topology shown in FIG. 2 or to a similar power supply topology but with one output capacitor per cell.
The efficiency of the power supply is improved relative to interlaced structures with no resonance. As a comparison, in a power supply with interlaced cells with no resonance, it is usual to obtain efficiencies of the order of 92% to 93% at full power. With resonance, these efficiencies at full power reach 96%, that is a gain of the order of 3% to 4%.
Usually, full power means the maximum power transferred by the converter, the limit being fixed by the nature of the components used.
In practice, a power supply does not always work at full power, that is to say that the current that is called for by the load can vary. This can depend on the application, because a power supply is designed for a certain power range, which allows it to be used for different applications. But then, all the applications do not have the same power requirements. Moreover, the power required by a given application may vary over time.
However, if the power supply does not work at full power, the gain in efficiency obtained with the interlaced structure with resonance is less, and is even reversed. This can be explained by the fact that a resonant circuit that comprises by definition resonant elements—resonance inductor and resonance capacitor—, is the source of specific power losses. The importance of these losses depends only on the level of output voltage that will charge the resonance capacitor. The energy handled by the resonant circuit depends specifically on the output voltage which charges the resonance capacitor. This output voltage modifies the energy stored by the resonance capacitor and therefore modifies the energy that is handled by the resonant circuit of the cell. These losses are independent of the power handled by the cell. It can therefore be understood that, depending on the conditions of use of the power supply, these losses of the resonant elements are of greater or lesser importance.
To illustrate this problem, the table inserted below shows the efficiency η of a given power supply with n interlaced cells with a resonant circuit (n=3), as a function of the power handled by the power supply.
The power P handled by the power supply typically depends on the required output level Vs and on the current Is called for by the output load, that is to say on the conditions of use of the power supply.
The table shows the efficiency η of the power supply corresponding to the ratio between the output power and the input power as a function of the level of output voltage required Vs, on the level VE of input voltage, and on the power P handled by the power supply.
P (kilowatts)VE (volts) VS (volts) η (percent)2 kW200 V360 V90.5%300 V91.9%245 V94.5%5 kW170 V360 V94.4%300 V95.6%245 V96.3%8 kW140 V360 V95.2%300 V95.6%245 V95.9%9 kW140 V360 V  95%300 V95.5%245 V  96%
In this table, for the power supply taken as an example, with which these figures have been obtained, the maximum power is 9 kW.
It can be seen in this table that the efficiency varies with the conditions of use: the efficiency deteriorates with a lower handled electric power. Notably, at a low power handled by the power supply, 2 kW in the example of the table, the efficiencies become lower than those of the power supplies with no resonance.
Although the level of loss of all the resonant circuits of the converter can be limited to levels of the order of 1% to 2% of the total power of the converter by using efficient capacitors of the polypropylene type with low serial resistance armature and planar inductors, a drop in efficiency is still observed when the power supply does not work at full power. Typically, for fixed losses in the resonant elements corresponding to 1.5% of efficiency at full power, these fixed losses correspond to a drop in efficiency of 4.5% when the converter works at a third of its power.